I second Jim's advice. Try a larger prop first and if that doesn't get you where you need to be you may have to start changing the rocker spring under the metering diaphragm until you get it right. There are a number of them available; they are typically sized for a particular carb/engine combo and they control the delivery curve to an extent. (What they actually control is the pop-off pressure on the regulator) That's why there are so many Walbro's with the same venturi size. They are custom calibrated to the specific engine the were intended for and also for the load the engine was designed to operate at. Change the load (think prop size here) and the calibration changes accordingly.